BACKGROUND AND OBJECTIVE: Evidence of short-term exposure to particulate matter with an aerodynamic diameter ≤1 μm (PM METHODS: In this time-stratified case-crossover study, we assigned cases who had been admitted to hospital for RDs in Guangdong, China between 2016 and 2019. Exposure to PM RESULTS: A total of 408, 658 hospital admissions for total RDs were recorded in the study period. Each 10 μg/m CONCLUSION: Our results support that PM